A successful virus infection involves entry into the cell; uncoating, expression and replication of the genome; assembly and release of infectious virus particles; and defense against specific and non- specific host immune mechanisms. Combined genetic, biochemical, electron microscopic, and immunological approaches are being used to investigate these complex processes. Two genetic approaches have been developed and applied to the study of vaccinia virus assembly. For essential genes, the elements of the E coli lac operon were used to construct inducer-dependent conditional lethal mutants of vaccinia virus. Repression of the D13L gene, which encodes a 65,000 Dalton protein, prevented the formation of the immature spicule-coated viral envelope and mimiced the action of the drug rifampicin. Repression of the F18R gene allowed virion morphogenesis to proceed up to the stage of differentiation of the virion core. The second approach, gene knock-out, was used to investigate the function of the outer viral envelope and the release of virions from cells. Deletion of the viral gene encoding the VP37 envelope protein did not affect the formation of intracellular infectious virus but prevented the membrane wrapping of virions and their fusion with the plasma membrane. Evidence was obtained for an additional post-budding step in virion release. With some strains of vaccinia virus, enveloped virions remain attached to the surface of cells but can be released with trypsin treatment. To evade the host immune system, vaccinia virus encodes a variety of proteins. We demonstrated that the complement control protein (VCP), the secreted product of the C2IL gene, is able to prevent complement- enhanced antibody-mediated neutralization of infectious virus. These data demonstrated that VCP blocks the alternative, as well as the classical, pathway of complement activation. A mutant virus containing a knock-out deletion of the C2IL gene was highly attenuated when injected into the skin of animals. The basis for the interferon- resistance of vaccinia virus was investigated. The product of the K3L gene was found to inhibit the double-stranded RNA activated protein kinase and to thereby prevent the interferon induced phosphorylation of the alpha subunit of eukaryotic initiation factor 2. Evidence against the possibility that vaccinia inhibits host protein synthesis by degrading or causing the underphosphorylation of components of the cap- binding translation initiation factor complex eIF-4F was obtained.